Carnot Efficiency 2: Reversing the Cycle
Seeing how we can scale and or reverse a Carnot Engine (to make a refrigerator)

Carnot Efficiency 3: Proving that it is the most efficient
Proving that a Carnot Engine is the most efficient engine

Heat scavenging
Stealing the heat
Energy: The idea of recycling paper, glass, metal and plastics has become commonplace. New technologies allow heat to be recycled, too

Mar 4th 2010 | From The Economist print edition

“By constructing a computer rack similar to that used in the office test, the researchers were able to provide the greenhouse with badly needed heat. A short while later, the rack was joined by three more racks that today provide the greenhouse with enough heat to cut its gas bills by $15,600 a year—while simultaneously saving Notre Dame $38,000 in cooling costs.

…

Another way to recycle heat that is being explored is to capture infrared with photovoltaic cells similar to those used in solar panels. Photovoltaic cells depend on packets of light (photons) knocking electrons free from atoms. They then employ the electrons so liberated to create a current. Photovoltaic cells are usually most responsive to photons in the visible and ultraviolet parts of the spectrum, but they can also respond to high-frequency infrared photons. Objects at a temperature of 1,000-1,500ºC produce plenty of such photons.

But only those that are travelling at a near-perfect right-angle to the surface of the hot material can escape and travel outwards. Photons travelling at any other angle within the material are reflected back inside when they reach the surface. As a result, photovoltaic cells placed near hot objects have only been able to generate around 0.02 watts per square centimetre. By contrast, photovoltaic cells absorbing sunlight can produce about 20 watts per square centimetre, provided the light is carefully concentrated using mirrors.

…

So Dr Hagelstein and his colleagues changed the design of the cell, adding tiny metal wires to the usual sandwich of semiconductor materials in order to pick up the liberated electrons and allow them to be carried off to create an electric current. Although the new device is still at an experimental stage, the team’s calculations, published in a paper in the Journal of Applied Physics in November, suggest that it could convert heat to electricity at a rate of 100 watts per square centimetre. Installed on a laptop, it could recycle heat from the microprocessor and extend running time by around 20%. One way or another, it seems likely that the abundant reservoirs of waste heat are about to be tapped.”

Why the corona is so much hotter is still not precisely known:
http://en.wikipedia.org/wiki/Corona#Coronal_heating_pro blem

The surface of the sun is only about twenty times hotter than the
average surface temperature of the Earth: about 6,000°K compared with
about 288°K.

People thinking in Celsius would likely get the ratio wildly wrong.

A heat engine makes a hot place and a cold place more similar, and makes energy.

A heat pump makes a hot place and a cold place more different, and requires energy.